[petsc-users] SNES line search

[Jed Brown]

On Wed, Oct 17, 2012 at 2:02 PM, Shiyuan Gu <sgu at anl.gov> wrote:

> Hi,
>     I am using petsc-3.3-p2.  For my particular equation, for the first
> few iterations, the residual norm is effectively reduced and the solution X
> is very close to the exact solution (the residual norm is e-17). Then in
> the next iteration, SNES_KSPSolve(....) returns a newton step very close to
> zero which is correct, but the line search after it fails, and the function
> SNESLSCheckLocalMin_Private(...) is called. In the function
> SNESLSCheckLocalMin_Private(),
> the solution vector X is rewritten to X=J^T*F where J is the Jacobian and
> F is the residual. Since F is very close to zero, X is also zero which is
> wrong.
> Is there a way to skip the line search?(if the line search is not
> performed, there would be no problem for this particular equation).  How
> should we handle this situation?
>

You can turn off the line search with -snes_linesearch_type basic, but I
recommend setting an appropriate atol and stol instead, to prevent
attempting to solve the system more accurately than machine precision when
given a good initial guess.


> Also, after the solution vector X is rewritten by
> SNESLSCheckLocalMin_Private(), the new residual  norm is not printed on
> screen (which is huge for this particular problem).  Since the residual
> norm printed on the screen is going to machine accuracy,  users might think
> that the SNESSolve converges nicely and the returned solution is correct.
> This seems a bit misleading in my opinion.
>

Are you checking -snes_converged_reason?


>
> Thanks.
>
> Shiyuan
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20121017/7ff592f6/attachment.html>